Daniel S. Miles

Applied Physiology of Ice Hockey

SummaryToday’s elite hockey players are physically bigger and have improved levels of physiological fitness when compared with their predecessors. Correspondingly, previous ice hockey studies that have become widely referenced may have little relevance to current players and the way the game is presently played.A great need exists to apply exercise science to the game of ice hockey. Although much has been written about the physiology of ice hockey, there is little information based on well controlled studies. Particularly, there is a paucity of knowledge concerning optimal training schedules, training specificity, recovery profiles and seasonal detraining. Moreover, the reports that do exist have attempted to make comparisons across all levels of skill and talent. Thus, fundamental questions remain as to actual physiological exercise response and specialised training programmes for ice hockey players, particularly at the elite level.There is a demand for new properly designed experiments to find answers pertaining to the appropriate training methods for today’s ice hockey players. Future research directions should consider the relationships between performance and such variables as neuromuscular skills, strength, power, peripheral adaptations, travel, hydration, detraining and sport-specific training programmes. Incidence and severity of injury among ice hockey players in relation to fatigue and fitness must also be investigated. Much of the information currently used in ice hockey will remain speculative and anecdotal until these studies are conducted.

Cardiovascular responses to upper body exercise in normals and cardiac patients

This review summarizes and contrasts the cardiovascular responses elicited during dynamic upper body exercise (UBE) with those associated with lower body exercise (LBE). Information was obtained from studies which utilized arm-crank and/or cycle ergometers. At any given submaximal oxygen uptake (VO2), cardiac output (Q) is similar for UBE and LBE; however, heart rate (HR) is higher and stroke volume (SV) lower during UBE. Peripheral resistance and systolic and diastolic blood pressure are greater during UBE. Maximal Q, HR, SV, workload, and VO2 are less for UBE. As observed with healthy individuals, cardiac patients experience greater physiological stress for any given VO2 during UBE. UBE offers a satisfactory but perhaps not equivalent alternative to LBE for evaluation of angina and ischemic responses to exercise. The central and peripheral responses to either upper or lower body exercise appear to be independent of the muscle mass but directly related to the ergometer specific relative exercise intensity. The control mechanisms which govern these responses appear to be a centrally mediated activation of medullary centers coupled with a chemoreflex arising in the exercising skeletal muscle.

Comparison of two impedance cardiographic techniques for measuring cardiac output in critically ill patients.

The purpose of the present study was to compare cardiac output (Q) values obtained by both the Kubicek (MIC) and Sramek (NCCOM3) impedance cardiographic techniques with thermodilution (TD) in critically ill patients. The two impedance techniques were also compared in normal subjects. Seven healthy subjects and ten ICU patients were enlisted in the study. Three Q measurements were made in each subject. In the ICU patients, there were no significant differences in Q values as measured by TD (6.6 L/min), MIC (6.3 L/min), and NCCOM3 (6.4 L/min). Both MIC and NCCOM3 Q values were comparable to TD in patients. In normals, however, the NCCOM3 estimated larger values for Q than did the MIC (NCCOM3, 9.2 L/min; MIC, 6.2 L/min). Q values obtained with MIC in normals were comparable with published values for supine normals. Thus, the two techniques agreed in the patients but not in the normals. The reasons for these results are not obvious from the data, but are attributable to the measurements by the NCCOM3. Because of this, caution is suggested when interpreting absolute Q values obtained by the NCCOM3.

Impedance cardiography fails to measure accurately left ventricular ejection fraction

The purpose of this study was to describe the technique proposed to measure left ventricular ejection fraction (LVEF) with the impedance cardiogram and to compare these values with those measured by radio-nuclide angiocardiography. Characteristics (mean ± SE) of the healthy control group were: age, 32 ± 3 yr; weight, 75 ± 6 kg; and height, 177 ± 3 cm. Characteristics of the patient population of 46 men and 49 women were: age, 63 ± 1 yr; weight, 74 ± 2 kg; and height, 170 ± 1 cm. LVEF was measured by impedance (ZEF) and multiple-gated scans (MEF) while in the supine position. The control group ZEF averaged 72% (range 67% to 78%) and the MEF averaged 71% (range 65% to 77%). There were no differences between the average ZEF (56 ± 1%) and MEF (53 ± 2%) in the patients. Correlations, however, between ZEF and MEF were unacceptably low for the several clinical populations within this group (-0.17 to 0.16). Furthermore, MEF correlated well with regional wall motion (r = .84) while ZEF did not (r = .00). Subdividing the patients according to heart function as determined by regional wall motion failed to improve the correlation between MEF and ZEF. The use of a previously published regression equation to predict LVEF from the systolic time interval ratio of pre-ejection period/left ventricular ejection time derived from the impedance cardiogram also proved ineffective. These data suggest that the previously proposed analysis of the impedance cardiogram to measure LVEF should not be used to make a clinical diagnosis. (Crit Care Med 1990; 18:221)

Application of impedance cardiography during exercise

Impedance cardiography has been used over the last 30 years to measure stroke volume on a beat-by-beat basis. Cardiac output has been successfully measured with either upper or lower body exercise during light or moderate workloads. With strenuous exercise, movement artifacts severely limit the acquisition of a quality impedance cardiogram. Advances in computer technology and signal conditioning techniques have created the next generation of impedance cardiograph systems. The purpose of this study was to evaluate such a system, the noninvasive continuous cardiac output monitor (NCCOM3-R7), at rest and during submaximal upright cycle exercise. In addition, the relationships between thoracic impedance (Z(o)), first derivative of the change in thoracic impedance (dZ/dt) and posture were evaluated using the NCCOM3-R7 and the Minnesota impedance cardiograph 304B (MIC). Twenty-eight healthy men and women participated. The Z(o) progressively increased when moving from the supine to seated to standing position with both instruments. However, the NCCOM3-R7 yielded lower Z(o) values and higher dZ/dt values compared with the MIC for all postures. Z(o) and dZ/dt values appear to be dependent upon factors such as posture, gender, electrical current, and characteristics of the instrumentation. Exercise cardiac output values seemed reasonable for most subjects, although population subsets exist where the accuracy must be questioned. The general consensus supported by the impedance literature and reaffirmed by the present observations is that impedance cardiography provides a reasonable estimate of the directional changes in stroke volume and cardiac output during exercise and can be used to monitor changes in thoracic fluid balance. As this technology evolves and is further refined, it will undoubtedly play an increasing role in environmental medicine, exercise stress testing, cardiac rehabilitation, and sports medicine.

Noninvasive assessment of cardiac output by impedance cardiography in the newborn canine

Currently, critical care monitoring of cardiac function in the newborn human consists mainly of measuring heart rate and BP. A noninvasive technique for assessing cardiac output routinely in the critically ill neonate would facilitate clinical management. Impedance cardiography (IC) is a noninvasive technique which measures stroke volume on a beat-by-beat basis. This study compared cardiac output as measured by thermodilution (TD) to that measured by IC in seven canine pups 6 to 7 days old weighing 0.66 to 0.86 kg. Cardiac output was altered by the withdrawal and reinfusion of blood. There were no significant differences between the two methods for either the absolute value of cardiac output (r = .96) or the percent change in cardiac output (r = .97). Coefficients of variation were 3.0% for TD and 3.6% for IC. These results indicate that IC can be used to assess serially cardiac function in the newborn.

Modified head-up tilt test for orthostatic challenge of critically ill patients

The purpose of this study was to assess the cardiovascular response to a modified head-up tilt test for use with the bedridden, critically ill patient. The cardiovascular responses of seven normals and ten critically ill patients to 45 degrees head-up tilt with the legs horizontal (0 degrees) were analyzed. Stroke volume index (SI) and cardiac index (CI) were measured with thoracic electric bioimpedance (TEB). The baseline TEB (Z0) was measured to monitor fluid shift out of the thorax during tilt. BP was measured and systemic vascular resistance index (SVRI) was calculated. Normals responded to the modified head-up tilt with a decreased SI (59 to 44 ml/m2 and CI (3.7 to 2.8 L/min.m2), and an increased SVRI and Z0 (25.5 to 27.9). As a group, the patients showed no significant change with tilt. However, individual analysis revealed a heterogeneous response by the patients. Those patients who demonstrated a caudal shift of blood (increased Z0) had decreased SI. Those with no indication of a caudal shift of blood, presumably due to decreased venous compliance, did not change SI. Thus, this type of modified head-up tilt can be used in the ICU to study more intensely cardiovascular function and control in the bedridden subject.

Cardiothoracic variables measured by bioelectrical impedance in preterm and term neonates.

ObjectiveTo report the range of normal values for impedance-derived cardiac output, stroke volume, and the baseline transthoracic impedance in the healthy preterm and term neonate over the weight range generally found in the intensive care nursery. DesignProspective, case-referent study. SettingUniversity medical center special care and term nurseries. PatientsTwenty-seven preterm and 25 term newborns with no evidence of cardiovascular problems. InterventionsWe determined the values for impedance cardiac output and stroke volume to be used as reference values. Also measured was the baseline transthoracic impedance, a number that reflects the air/fluid ratio of the thorax. Measurements and Main ResultsStroke volume and stroke volume index were, respectively, 2.0 ± 0.8 (SD) mL and 1.4 ± 0.5 mL/kg for preterm infants, and 5.0 ± 2.0 mL and 1.6 ± 0.7 mL/kg for term neonates. Cardiac output and cardiac index were, respectively, 304 ±114 mL/ min and 214 ± 68 mL/min-kg for preterm newborns, and 648 ± 244 mL/min and 205 ± 78 mL/min-kg in term newborns. These values compared favorably with published values utilizing other techniques for these populations. Both cardiac output and stroke volume were linearly correlated to body weight, being largest in the heavier neonates. Transthoracic impedance values were 42.7 ± 9.0 ohms and 6.7 ± 1.7 ohms/cm for preterm infants and 32.3 ± 4.3 ohms and 3.9 ± 0.6 ohms/cm for term infants.Transthoracic impedance and transthoracic impedance/cm values were correlated negatively to body weight and were curvilinearly related to body weight. ConclusionsThese values for transthoracic impedance and transthoracic impedance/cm are the first reported using the standard electrode lead configuration in neonates.